It is well known that the resolution of probe microscopy or probe writing, such as atomic force microscopy (AFM) imaging and probe-based information storage systems are determined by the sharpness, size and shape of the probe tip. For general background, see the following articles; Rugar, et al, “Atomic force microscopy”, Phys. Today 43(10), 23-30 (1990), Noy, et al, “Chemical force microscopy”, Annu. Rev. Mater. Sci. 27, 381-421 (1997), Hansma, et al, “Biomolecular imaging with the atomic force microscope”, Annu. Rev. Biophys. Biomol. Struct. 23, 115-139 (1994), Shao, et al, “Progress in high resolution atomic force microscopy in biology”, Quart. Rev. Biophys. 28, 195-251 (1995), G. K. Binnig, et al, U.S. Pat. No. 5,835,477, Nov. 10, 1998, “Mass-Storage Applications of Local Probe Arrays,” and an article by P. Vettiger, et al, “Ultrahigh Density, High-Data-Rate NEMS-Based AFM Data Storage System,” J. Microelectron. Eng. 46, 11-17 (1999).
Typical commercially available AFM probe tips are made of silicon or silicon nitride (Si3N4) which is microfabricated into a pyramid configuration. Such probes have a typical tip radius of curvature in the ˜50 nm regime, thus exhibiting a limited lateral resolution, and their rigid pyramid shape does not allow easy access to narrow or deep structural features.
Utilizing the advances in carbon nanotube science and technology, a potential new breakthrough in probe technology, i.e., employing a “thin-probe-on-pyramid” configuration was presented, for example, see U.S. Pat. No. 6,716,409, “Fabrication of nanotube microscopy tips” issued to Hafner, et al. on Apr. 6, 2004, U.S. Pat. No. 6,401,526, “Carbon nanotubes and methods of fabrication thereof using a liquid phase catalyst precursor” issued to Dai, et al. on Jun. 11, 2002, articles by Dai, et al., “Nanotubes as nanoprobes in scanning probe microscopy”, Nature 384, 147-150 (1996), by Colbert, et al, “Growth and sintering of fullerene nanotubes”, Science 266, 1218-1222 (1994), by Wong, et al, “Carbon nanotube tips: High-Resolution probes for imaging biological systems”, J. Am. Chem. Soc. 120, 603-604 (1998), by Nishijima, et al, “Carbon nanotube tips for scanning probe microscopy: preparation by a controlled process and observation of deoxyribonucleic acid”, Appl. Phys. Lett. 74, 4061-4063 (1999), Stevens, et al, “Carbon nanotubes as probes for atomic force microscopy”, Nanotechnology 11, 1-5 (2000), by Yenilmez, et al, “Wafer scale production of carbon nanotube scanning probe tips for atomic force microscopy”, Appl. Phys. Left. 80, 2225-2227 (2002), and by Minh, et al, “Selective growth of carbon nanotubes on Si microfabricated tips and application for electron field emitters”, J. Vac. Sci. Technol. B21(4), 1705-1709 (2003). Carbon nanotubes are typically grown by using chemical vapor deposition (CVD) technique in which hydrocarbon gas is decomposed at high temperature often assisted by DC or RF plasma.
The long and slender geometry of carbon nanotubes (high aspect ratio) offers obvious advantages for probing narrow and deep features. The elastically compliant behavior of high aspect ratio nanotubes is also advantageous. Even when the stress encountered by the nanotube probe reaches beyond a critical force, the nanotube can elastically buckle and recover to accommodate the strain, thus limiting the maximum force exerted onto a sample being imaged by the AFM probe. This is particularly advantageous when the samples being examined are mechanically soft or fragile such as in the case of biological surfaces.
In these prior art processes the attachment of a carbon nanotube onto an AFM probe tip is accomplished by several different means, for example, using acrylic adhesives under optical microscope, carbon deposition in a scanning electron microscope (SEM), or electric arc discharge technique. In-situ growth of carbon nanotubes directly on AFM tips were also reported in US patents by Hafner, et al. and Dai et al., and articles by Yenilmez, et al. and by Minh, et al. cited above.
With the trend of miniaturization and nanoscale devices for semiconductors and electronic devices, an inspection of the fabricated devices and a critical control of the three-dimensional dimensions and features is essential. Atomic Force Microscopy (AFM) or Scanning Probe (or Force) Microscopy (SPM, SFM) is a versatile technology for measurement and inspection of the surface of semiconductors during fabrication as the vertical and lateral resolution can be nanometer- or subnanometer-scale. Conventional AFM applications for topological measurements are on largely horizontal or non-reentrant surfaces, and are limited by the pyramid (or parabolic) shape of the probe tip and the AFM's scanning control algorithm.
A special type of AFM called Critical Dimensional AFM (CD-AFM) is designed to allow scanning of sidewall and enable measurements of vertical and reentrant surfaces of the sample, for example, in trenches or via holes of semiconductor substrates and devices. Two key technologies for CD-AFM are:
1. Advanced scanning algorithms (CD modes)
2. Special probe shapes (CD probes).
A CD probe usually has a flared apex region. The lateral edges of the tip enable accurate profiling of vertical or reentrant surfaces. Current CD-probes are primarily made of silicon material and the tip width is in the range of 30 to 1000 nm. See U.S. Pat. No. 5,171,992, Joachim G. Clabes et al., “Nanometer scale probe for an atomic force microscope, and method for making same”, Dec. 15, 1992, U.S. Pat. No. 5,242,541, Thomas Bayer et al., “Method of producing ultrafine silicon tips for the AFM/STM profilometry”, Sep. 7, 1993, U.S. Pat. No. 5,382,795, Thomas Bayer et al., “Ultrafine silicon tips for AFM/STM profilometry”, Jan. 17, 1995, Yves Martin and H. Kumar Wickramasinghe, “Method for imaging sidewalls by atomic force microscopy”, Appl. Phys. Lett. 64 (19), 9 May 1994, Yves Martin and H. Kumar Wickramasinghe, “Toward accurate metrology with scanning force microscopes”, J. Vac. Sci. Technol. B 13 (6), November/December 1995 H. Liu, M. Klonowski, D. Kneeburg, G. Dahlen, M. Osborn, T. Bao, “Advanced AFM Probes: Wear Resistant Designs”, J. Vac. Sci. Technol. B 23 (6), pp 3090-3093, 2005.
Silicon-based CD-AFM probes have not shown the capability to support 45 nm node AFM metrology (which means a capability to be inserted into 45 nm trenches or via holes and carry out sidewall scanning). Since such CD-AFM probes which can reliably perform smaller-node metrology below 45 nm has not been demonstrated, there is a need to develop new, non-silicon probes capable of such a task.
As the dimensions of the sample shrink (for example, semiconductor CMOS devices and magnetic heads for hard drive disks), the fabrication of silicon-based CD probes becomes increasingly challenging. Moreover, the wear rate of the tips accelerates as the size shrinks. For instance, in some semiconductor applications the entire flare of a silicon tip may be worn away while scanning a single measurement site. In contrast, carbon nanotube (CNT) tips have been shown to have lifetimes easily exceeding silicon tips by an order of magnitude. See an article by Liu et al., Proc. of SPIE Vol. 6152, 61522Y (2006). A second, key advantage of a CNT tip is that the higher strength (Young's modulus) of the CNT vs. silicon allows tip lateral stiffness to be maintained. CD tip lateral stiffness is a growing problem for silicon tips as the tip size shrinks and directly results in measurement error and the tip actually “sticking” to the scanned feature. Consequently, CD probe tips made of non-silicon material is preferred.
This invention discloses new and novel, sidewall tracing AFM probes with such desirable characteristics and is also capable of scanning nanoscale features in the small trenches or via holes. Various embodiments, various probe configurations, alternative fabrication methods, various applications, and the various modes of uses are described.
Scanning probe microscopy (SPM) such as atomic force microscopy (AFM) has been an important and powerful technique for resolving nanoscale features, and thus has been utilized for various scientific, engineering, and biological applications. The key component of SPM is the probe tip, as the resolution of SPM imaging is determined by its sharpness, size and shape. See articles by G. Reiss, et al, “Scanning tunneling microscopy on rough surfaces: tip-shape-limited resolution”, J. Appl. Phys. 67, 1156 (1990), and by J. E. Griffith et al, “Scanning probe metrology”, J. Vac. Sci. Technol. A10, 674 (1992). Typical commercially available SPM probe tips are made of silicon or silicon nitride microfabricated into a pyramid configuration. Such probes are often easily broken or worn out during long time operation. They also generally exhibit a limited lateral resolution, and their rigid pyramid shape does not allow easy access to narrow or deep structural features.
Carbon nanotubes (CNTs) have attracted much attention due to their various interesting physical and chemical properties. The high aspect ratio geometry and the nano-scale diameter of the CNT offer obvious advantages for imaging as an AFM probe. Moreover, due to its good mechanical flexibility, such a CNT probe is also suitable for studying soft matters such as biological samples with minimal damage.
Carbon nanotubes (CNTs), either single wall carbon nanotubes (SWNTs) or multiwall nanotubes (MWNTs) can be grown in a controlled manner using chemical vapor deposition (CVD) processing. Carbon nanotubes with graphene walls parallel to the axis of the nanotube as well as those with graphene walls at an angle to the axis of the nanotube can be grown. The latter type of carbon nanotubes, sometimes called nanofibers, often still have a nanoscale tube configuration, and hence will be referred throughout this application as nanotubes. Vertically aligned, periodically spaced MWNTs can be grown in a controlled manner using DC-plasma enhanced CVD process using an applied electric field. See V. I. Merkulov, et al, Appl. Phys. Lett. 80, 4816 (2002), J. F. AuBuchon, et al, Nano Letters 4, 1781 (2004).
In addition to carbon nanotubes, there are other types of nanowires which may also be useful as the nanoprobes. Some examples include silicon nanowires with gold-rich catalyst particle at the tip, see an article by Morales et al, Science 279, 208 (1998), and ZnO nanowires with gold-rich Au—Zn catalyst particle at the tip, see articles by Huang, et al, Science 292, 1897 (2001) and by Yang et al, Advanced Functional Materials 12, 323 (2002).
There have been several approaches developed for fabrication of CNT based probes. Most approaches are based on attaching CNTs (mostly multiwall nanotubes) on commercial pyramid tips by acrytic adhesive, electric field, arc welding, magnetic field and liquid phase dielectrophoresis. See articles by H. Dai, et al, Nature 384, 147 (1996), H. Nishijima, et al, Appl. Phys. Lett. 74, 4061 (1999), by R. Stevens, et al, Appl. Phys. Lett. 77, 3453 (2000), by A. Hall, et al, Appl. Phys. Lett. 82, 2506 (2003), and by J. Tang, et al, Nano Lett. 5, 11 (2005). These methods are operated manually and are time consuming. The attachment angle, the number of CNTs attached, and adhesion strength are not always controllable. A direct growth of CNTs with catalyst particles or catalyst film coating on Si tips by thermal CVD has also been reported. See an article by I. C. Chen et al., “Extremely sharp carbon nanocone probes for atomic force microscopy imaging”, Appl. Phys. Lett. 88, 153102 (2006).
In order to allow metrology and inspection of sidewalls in the trenches and via holes of semiconductor devices, the tip of the AFM nanoprobe has to be either bent or be thicker than the rest of the probe length. Most of these nanotubes or nanowires are equi-diameter or tapered down sharp along the length of the wire toward the tip, and hence they do not allow sidewall tracing capability if used as the AFM probe. The possibility of utilizing sharply bent carbon nanotubes for sidewall metrology has been disclosed in prior art patent applications as discussed earlier. The use of bent nanotubes generally require more complicated CVD deposition processing.
Therefore it is desirable to find a unique, alternative and simpler fabrication processing for sidewall tracing nanoprobes, for example, by altering the tip shape of already fabricated nanowires or nanotubes in such a way that the diameter or width of the very tip of the probe is wider, greater than the diameter of the supporting wire beyond the catalyst particle at the tip. This invention discloses such processing techniques to allow fabrication of unique simple, reliable and protection-layer-free techniques for fabricating a single SPM probe on the cantilever and unique probe tip structures by utilizing the unique sidewall tracing nanoprobes.